An example of a background locking device will be described with reference to FIGS. 6 and 7. FIG. 6 is a view showing a locked state of the background locking device, and FIG. 7 is a view showing an unlocked state of the background locking device.
The locking device is attached to each of a moving-side member and a fixed-side member. The moving-side member is, for example, a seat, a door, a trunk lid or an engine hood, which is provided to be rotatable relative to a floor of a vehicle. The fixed-side member is, for example, a vehicle body. The locking device is a device that is capable of suppressing the relative movement between the moving-side member and the fixed-side member.
The locking device includes a base 1 and a striker 3. The base 1 is provided in one of the moving-side member and the fixed-side member, and the striker 3 is provided in the other one. The base 1 is provided with a groove 1a into which the striker 3 is able to enter.
A hook 7 is rotatably provided in one side of the base 1. A pawl 9 is rotatably provided in the other side of the base 1. The groove 1a is provided between one side and the other side of the base 1. The hook 7 is formed with a groove 7a into which the striker 3 is able to enter. The hook 7 is formed with a convex portion 7b. 
The pawl 9 is formed with a concave portion 9a with which the convex portion 7b of the hook 7 can be engaged. The concave portion 9a has a shape that is spread toward its opening.
A spring 11 is caused to exert a force on the hook 7 and the pawl 9. One end of the spring 11 is locked to the hook 7, and the other end thereof is locked to the pawl 9. The spring 11 urges the hook 7 in an unlocked position direction (in the drawing, a direction of an arrow UL). The spring 11 urges the pawl 9 in a direction (in the drawing, a direction of an arrow K) in which the convex portion 9a is engaged with the concave portion 7b of the hook 7.
In the state shown in FIG. 6, the hook 7 is in a locked position where the striker 3 is prohibited from being detached from the groove 7a. This state is a locked state where the concave portion 9a is engaged with the convex portion 7b, and the rotation of the hook 7 is thus prohibited.
In the state shown in FIG. 7, the engagement between the concave portion 9a and the convex portion 7b is released. This state is an unlocked state where the hook 7 is in an unlocked position and where the striker 3 is able to enter the groove 7a. 
Next, an operation of the locking device configured as described above will be described.
When the pawl 9 is rotated, from the locked state shown in FIG. 6, in a direction opposite to the direction of the arrow K against an elastic force of the spring 11, the engagement between the concave portion 9a and the convex portion 7b is released. The hook 7 is further rotated, by the elastic force of the spring 11, to the unlocked position where the striker 3 is able to enter the groove 7a. In this way, the locking device becomes the unlocked state shown in FIG. 7, where the convex portion 7b is abutted against a portion other than the concave portion 9a. 
In the unlocked state shown in FIG. 7, the hook 7 is rotated in the locked position direction against the elastic force of the spring 11 when a surface in the vicinity of an opening of the groove 7a of the hook 7 is pressed by the striker 3. Further, the convex portion 7b of the hook 7 and the concave portion 9a of the pawl 9 are engaged, and the hook 7 is thus prohibited from rotating in the unlock direction. Then, the striker 3 becomes the locked state where the striker 3 is not able to be detached from the groove 7a of the hook 7 (e.g., see Patent Literature 1).